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Making a first mirror: 10" f6

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#26 davidc135

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Posted 22 November 2019 - 08:01 AM

OK, this is great! I've been bashing about in the dark. I'll spend the weekend re configuring my work area to support testing in doors. At this stage I want to be able to test as I go. Looking at the Stellafane Ronchi Table "Curves into Center" Inside ROC the edge is under corrected, this supports the drawing by Dave above being a TDE?

 

Thank you

Chris

If the edge is under corrected it's focusing too short and a tue edge also focuses too short. Because this zone focuses short, on the knife edge side it appears dark. As in dogbiscuit's drawing.   David


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#27 Chris Adamczyk

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Posted 25 November 2019 - 04:22 PM

I have attached a picture of my lap after about 10 minutes of work, and a Ronchi inside the ROC. I would like to know if I am close enough to go to parabolizing? I plan on following David Harbours book. Trying to figure out the shadows at the ROC I see a bright ring at the edge and all my shadows were one sided?

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#28 dogbiscuit

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Posted 25 November 2019 - 08:03 PM

Shadows are one sided because the shape is spherical.  When moving away from the sphere you will begin to see shadows different on both sides.

Is the bright ring at the edge all around, or is it missing or significantly darker on one side? 

Bright all around indicates a good edge.

 

Ronchi shows edge better,   only slight turn up. Really quite good.

It looks to me like you used a different grating with more lpi, maybe a 133 lpi.  Is that correct?

 

The lap looks to be in good contact.

Don't know why you have such different spacing of the channels, especially that one channel that is very close to the adjacent channels.

I wouldn't do it that way, but maybe it will work ok.  Within the limits of what can be seen in the Ronchi it has made a pretty good sphere.

I hope you will be able to test in better conditions.  The poor quality of the Ronchigram could be hide surface roughness.  I guess the source of trouble is air movement from testing outside.

 

Tell us what you did since the last Ronchigram.

 

If you test by Foucoult with a Couder screen,  it makes testing much easier if the zone dimensions of the mask are made to have the same knife movement from each zone to the next.

For example, here is a 4 zone mask that would have 0.025" (0.635 mm) knife movement from each zone to the next.

Same knife movement between zones .025” for 10” f:6

Mirror diameter 254 mm
Clear aperture 250 mm

 

Mask zones  

                                               Carlin

                                              Method

Zone   R inner     R outer    R center 

  1        22.000       64.250     45.639     
  2        64.250       89.000     77.123     
  3        89.000     108.750     99.121     
  4      108.750     125.000    117.016

 

I show mask dimensions in mm because I find it easier to measure the layout of the mask with a mm scale.

This particular mask has inner and outer zone radii rounded to 1/4 mm because it is not to difficult to interpolate 1/4 mm increments between full mm scale rulings.

 

By having all target knife movements the same, you can know immediately which zones are over or under corrected relative to the parabola.

Don't need a program for figuring. Just need to be able to drive each zone as near as possible to the target knife movement.  Since the target is the same for each zone, it's easy to remember and easy to see by the actual knife movements from test readings what needs work.

 

Couder Mask program by Mark Holm can draw that mask out for you if you put those dimensions in.  The program can calculate mask dimensions for you but it won't get it to as precisely the same knife movements between zones as the mask design shown above.  You can edit the dimension in Couder Mask program and print the mask on card stock.  Guess for 10" you would need to print on two sheets and accurately join them.

 

Except for what I can't see about the mirror surface because of poor Ronchigram quality, and not seeing a Foucault image, the mirror figure looks ready for parabolizing.  I think you are safe to go ahead but you will need to have better testing conditions for accurate figuring.



#29 Chris Adamczyk

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Posted 25 November 2019 - 10:04 PM

I have been converting for inches to mm for ease of layout. At the ROC I saw a thin bright ring ALL around the edge using the Foucault test. Since the last Ronchi I did one session with the sub-diameter lap tested, liked what I saw and went to the full size lap, the Ronchi above was after 15 rounds of the barrel and tested with 133 lines per inch. I'll look at the Clouder Mask program, I was planning on using an Everest stick with curved reference wires per David Harbour, he uses 5 zones at 40,69,89,105 & 120 mm. But I'm open to suggestion. I don't want to rush at this stage.

 

I'm using a work shed I have in the back yard it's not possible to seal out all drafts but the images look much better than what I'm able to photograph with my phone.

 

Thank you for taking an interest in my struggles.

Chris



#30 dogbiscuit

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Posted 26 November 2019 - 12:35 AM

4 zones is enough for a 10" and will make testing your first mirror easier.

I forgot... typical printer card stock is 8 1/2 x 11"... doesn't have to be 10" each way.  So the mask can be printed on one sheet.

Here is what the Couder Mask 4 zone looks like, with some data printed on it for convenience, and central area behind the secondary grayed out just so you know what I'm saying about the shadow.

10inch5ZoneCouder.jpg

 

The very center is difficult to measure accurately, but it is behind the secondary, so there is an area of the mirror just a little smaller than the secondary that is not used.  No need to measure that.  With the secondary shadow in mind, I chose a convienient inner radius for the first zone that helped to make all target knife movements zone to zone to be the same.

 

The outer radii of the outer zone is just a little smaller than the full mirror radius to account for a bevel and leave out some of a possible turned edge that might cause difficulty finding a null for the outer zone.

 

 

The Couder Mask program numbers the first zone as 0.  I have renumbered to have the first zone as zone 1.

 

 

5 zones is ok, and I have dimensions for a good 5 zone Couder screen too.  :-)

 

250mm5zone

Mirror diameter 254 mm

Clear aperture 250 mm
 

Mask zones

 

                                                   Carlin

                                                  Method

Zone    R inner      R outer     R center
                                         
  1          17.750       57.000      39.868     
  2          57.000       79.000      68.443     
  3          79.000       97.000      88.230     
  4          97.000     111.250    104.247     
  5        111.250     125.000    118.225

 

For 10" f:6 0.02" (0.508 mm) knife movement between each zone.

 

Pinstick method might work ok.  I've done that before but came to like using a mask.

I'll let others work with you on that.  It's been more than a few years since I have done the pinstick thing.

 

See with the 4 zone mask in testing you get 4 zone reading.

There are 3 knife movements between those zones.

It is the knife movement that you want to know.

 

Assuming a moving source tester (light source and knife move together:

If zone 1 knife movement is nearest the mirror and knife 0.025" farther away from the mirror for each zone outward, the mirror is near perfect.

It is somewhat difficult to get all three movements perfect.  Get them as close as possible to the target.

 

If tester's light source is fixed knife movements would .05" between adjacent zones.

 

I have one comment (at least) on tester design.  The Stellafane tester does not account for your nose.  If you look at there tester design, you can have the knife and light source without the big "face board".   Modern LED's are small and a knife and light source unit can be small and mounted on a suitably shaped support so you can get your eye to it without touching it with any part of you face.  :-)  You should consider which eye you prefer to use for testing.  if you are left eyed you may want the knife and light source to the right side of the base.

 

I think it is safe to say you will have to build and use a tester to learn how you should have made it in the first place.  :-)

A skilled tester can do pretty well with a junk tester.  A beginner will have much trouble with anything less than the best. 

But it is not likely you will have the best, and it's part of learning, to manage with less than the best.

Don't let that worry you.  A 10" f:6 is not really all that difficult.  You should be able to do pretty well with a typical Mark 1 Foucault tester.

 

Perhaps a test tunnel of some sort can be fabricated to block drafts.

 

My first mirror was a 10" f:6.  Great views with that mirror.

 

You have a good sphere and edge, so time to parabolize.

Try 5 turns of MOT parabolizing strokes like this one.

Parab3fourths.jpg

This is a not the typical full size parabolizing stroke.

It's a little smaller so that hopefully the central crater that most beginners get with the larger size stroke will be avoided.

See how this one works and maybe later a larger one will be used.

 

It is mostly the overhang to left and right that add the correction.  It is added at the left and edges of the lap.

So it is the central parts of the W that add correction to the outer zone of the mirror and outer parts of the W that add correction to the mirror's center.

Pressure increases quickly with added overhang so the center tends to correct faster than the outer zone.

 

 

The forward and back overhang adds some but not as much as left and right overhang.

Notice on the left and right ends of the stroke the stroke direction has an increased lateral component.  This reduces the tendency of developing a central dimple or trenches near center.  Some of this would be in the secondary mirror shadow, but as a matter of pride you don't want this ugly stuff.

 

5 turns should be enough to see some movement toward the parabola and not enough work to make a big mess if it doesn't go right.

Do it like the drawing, starting on one side of the W, stroke to the other side, turn the mirror about 1/12 turn and take a similar size (give or take a little) around the barrel.  I go around the same way I turn the top disk, but I don't think it matters much.  Stroke the W back across the other way, turn, step, across the other way and so on.  Concentrate on doing it the same every time as you make the full turn of the mirror.

 

A 10" mirror is a nice size for fit to hands.  A good method is to place hands palm down with thumb tips touching at the center of the mirror.  Slight dampness of the hands will help grip the glass. Downward pressure  is evenly distributed over all of palm and finger contact with the glass.  You don't need to apply much pressure to get a grip for stroking.  I'd say about 10 or 12 pounds total. 

Don't change that even distribution.  The stroke diagram shows the path of the mirrors center over the lap.  Use you thumb tips as a marker of the mirrors center.  You could place a small piece of tape there too if you like.  The mirror will not tip over the edge of the lap as long as the center of the mirror doesn't go past the edge.  So even though you will be using where the center gets near the edge, you will not go past the edge, don't worry about it.

 

If it takes about 2 seconds for each stroke (forward and back = 1 stroke), the 5 stroke across the W will take 10 seconds.  12 times of that for a full turn of the mirror will be about 120 seconds or 2 minutes.  5 turns about 10 minutes for the entire work session.  Do only 5 turns.  Time it and let me know how long it actually takes you to do those 5 turns.  Don't do more.

 

Test and lets see what happened. When the result is seen maybe something else would be better.

 

There is a tendency to want to work more in one session. It can lead to trouble and much back tracking.  Small steps to the parabola.

 

 

 

 


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#31 dogbiscuit

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Posted 26 November 2019 - 09:50 AM

Concerning a Foucault tester:


The Stellafane tester.

https://stellafane.o...t/tester-1.html

Basically good but has knife in the middle of that large board, making it uncomfortable bending your face around to get your eye close enough without bumping your nose on the tester.  Every time you bump the tester you have upset the reliability of your knife reading.

So everything is generally ok except that big board holding the knife and light source.


Here is an example of typical first time Foucault tester very similar to design in Texereau's book "How to Make a Telescope".  In fact comparing to to the drawing in his book, it is exactly the same.  Same drawing with text in French.  

https://www.grattave...xereau/?lang=en

Fairly easy to build and works pretty good.  Similar to the Stellafane tester but without the nose bumper. 

 

Referring to the drawing, you can do away with the lamp house.  Just glue an LED to the knife edge and you have a slitless tester.

 

You could use a 1/4-20 screw as a micrometer (0.05" per revolution) to keep things inexpensive,  or use a micrometer.  The screw measures accurate enough.  Maybe a good idea to lap the threads a little to get smooth rotation.

 

You only need a short travel for testing but I recommend you make the travel 1 1/2" or 2".  That helps for accurate alignment of the tester to the optical axis. With the knife near the rear of its travel align to knife to null near outer zone of the mirror.  Push travel to near the forward limit and rotate the tester on the vertical axis to bring knife edge shadow to fill half of the mirror. Maybe a few iterations of this to get it just right.  Properly aligned, the full range of knife movement will be along the optical axis.  While testing you will not need to move the knife left or right much at all.

 

Not absolutely necessary but you might consider constructing  the base in a way that it can be clamped to the table when alignment to the optical axis is complete.  Some weight added to the top board will help stabilize it so it stays down on the ways.  Touch the micrometer very lightly when adjusting the knife position.  Set the tester and mirror at a convenient height for comfort when testing.  If you are uncomfortable you will rush the testing so you can be comfortable again.  :-)

 

Consider the eye that you use, the hand you will use to turn the micrometer.  The tester shown is better for the right eye.

You can flip the drawing left to right if you will be using your left eye.  The micrometer should be positioned so that with your eye to the knife the micrometer is clear of your chin and you can easily get your fingers to it.

 

These testers are good enough for testing your 10" f:6.

 

Now there is the mirror stand.......


Edited by dogbiscuit, 26 November 2019 - 10:01 AM.


#32 Chris Adamczyk

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Posted 26 November 2019 - 03:15 PM

I think I can modify my existing tester, I like the idea of glueing the LED to the knife tester. I'll also get a childs collapsible tunnel or AC ducting.

 

Thank you

Chris 



#33 Chris Adamczyk

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Posted 02 December 2019 - 09:17 AM

I did the 1st figuring yesterday after rebuilding my tester I ordered a dog agility tunnel to stop any drafts. It took be 10 minutes to complete 5 turns, I was very surprised that the time matched your estimate. I tried the Foucault test and could not see any shadow images so a used the 133 line Ronchi grating. I was very nervous doing the process, at the end of each W, I would turn the mirror clockwise and step to the right , I continued the W going back from the last end or should I have repositioned the mirror (not sure if that make sense). I didn't apply much more pressure than what was need to move the mirror.

 

ChrisOutside ROC 1st.jpg

Inside ROC 1st.jpg


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#34 Pinbout

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Posted 02 December 2019 - 03:10 PM

I'd do some figure 8's to smooth it out, nothing too overhanging in any direction.



#35 dogbiscuit

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Posted 02 December 2019 - 03:27 PM

I prefer doing a W from one side to the other, doing the turning and stepping at the sides of the W like you describe doing.

 

Ronchigram focus is not very good. At least I can see you did not dig crater in the middle.  That's good

For accurate focus I print  a sort of test pattern with a few thick black lines or large size letters and place it beside the mirror.  Light it well enough to see from the tester and focus on.  Turn the light out and take the Ronchigrams.

 

 

Can you work tool on top?  To work TOT the table surface needs to be flat and a pad between the mirror and table.
A medium density foam mat such as an anti-fatigue mat or yoga mat about 6 to 12 mm thick makes a good pad.
To keep the pad clean and dry it can be covered with a thin plastic sheet cut from plastic trash bags. The mirror is rotated in increments on the work table every 1 or 2 trips around the mirror.

 

 

I know you want to be stroking toward a parabola, so another 5 turns MOT of the same thing should be ok.
The drawing in post #30 shows 5 strokes across the W, and Unless otherwise stated I mean for the stroke to be done as drawn... 5 strokes. 
This time make it 7 strokes across the W. Those extra strokes should make the session take about 14 minutes give or take a little.

 

While you are waiting for the mirror to stabilize in temperature for testing, Answer these questions.

Are those Ronchi images inside or outside?
Are you sure those are with the 133 lpi grating?  Looks more like the 85 line grating.

What sort of pitch are you using?  Looks a bit like Gugolz.  If so, what grade.
What is your workspace temperature?


Edited by dogbiscuit, 02 December 2019 - 03:27 PM.


#36 Chris Adamczyk

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Posted 03 December 2019 - 08:48 AM

inside 2nd session.jpg I did the 7 leg W Stroke last night took 14 minutes, give or take forgetting to note the stat time. I definitely see progress, Ronchi images attached. I tried the Foucault test and was really a little lost, hopefully my 18" dia tunnel will arrive tomorrow. I will let the mirrors temperature equalize tonight and play around with the KE test somemore. outside 2nd session.jpg inside 2nd session.jpg



#37 Pinbout

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Posted 03 December 2019 - 09:14 AM

you need to label the images inside RoC or outside RoC. tongue2.gif

 

and now you added correction, but still zoney… that's why I like 8's to smooth.


Edited by Pinbout, 03 December 2019 - 09:15 AM.


#38 Chris Adamczyk

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Posted 03 December 2019 - 11:22 AM

sorry not sure how to edit the picture names, the second image about is Outside the ROC. I have been trying to stick with Vikings input rather than going with different points of view, I really like the feedback, I'm trying to be cautious in my approach.

 

Thank you

Chris 



#39 dogbiscuit

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Posted 03 December 2019 - 06:41 PM

That looks pretty good,

but don't understand the edge.  It looks as though there is some turn down in the outside Ronchi,  but looks like turn up in the inside Ronchi.

For now I'll assume the edge is somewhere between turned up and turned down.  :-)

 

A little correction showing in the center 60%

Little or no correction outer 40%

Yellow circle is about the 60% zone.

Red highlights bands bowing outward in center area inside 60% zone

Blue highlights nearly straight bands out side 60% zone

OutsideROCpost36.jpg

 

Correction lagging in outer zone is somewhat expected.  Not desired but what commonly happens.

Starting off with short sessions is a good idea (you've done that) so things can be done to fix things that aren't going exactly right before they turn into big problems.

 

Long strokes with more overhang to the side (like wide Ws) work farther from center.  Long strokes with less overhang are the ones that add correction to the outer zone. 

 

So a few turns with long strokes but not as much side motion, W not as wide as before, should help catch the edge up  with center correction.

So here is a .6 diameter long by .6 diameter wide W stroke.  Notice the higher density of strokes near center and decreasing density to sides of the W.


 

ParabOuterZone.jpg

 

For your 10" mirror maximum stroke length is 8" and width of the W is 6".  For this one 7 strokes across the W. 

The drawing shows the path of the center of the mirror over the lap.

I think 5 turns would be ok this early in figuring,  but in keeping with the idea of going slow and not letting things go wrong fast,  lets make it only 4 turns.

4 turns MOT

This should help correct the outer 40% without much change in the center 60%.  Center correction might go down a little.

After getting more correction in the outer zone,  The stroke is adjusted in stroke length, width of W, and stroke density/overhang attempting to get a balance that evenly corrects the entire mirror. Or a series of different size length and width Ws that concentrate work in different parts of the mirror can be done, mixing them in proportion to needed work in different zones.

 

Answers to those questions in my previous post helps me know what to expect and give useful advice

Let me know if you can work tool on top.  If you can, that is a very effective way to add correction in the outer zone.

If you can work TOT instead of the MOT stroke above I would suggest a TOT method of correcting the outer zone.


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#40 ccaissie

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Posted 03 December 2019 - 09:22 PM

That is clearly a fairly severe case.

 

You might need to go back to fine grinding to get close to a sphere before repolishing.

 

um, no.



#41 ccaissie

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Posted 03 December 2019 - 09:38 PM

 I was planning on using an Everest stick with curved reference wires per David Harbour, he uses 5 zones at 40,69,89,105 & 120 mm. But I'm open to suggestion. I don't want to rush at this stage.

 

Yes, I find the pinstick method great.  Rather than a stick with pins, I use the blue painters tape with little notches cut to mark the radii.  The diffraction effect on the notches lights them up, and watching for shadows to simultaneously cross the notches is very easy to judge.

 

I like that method because you set the knife edge to the correct distances per the usual formula, and then watch whether the shadows are shy or excessive over the pins/notches.  i.e over or over, respectively. 

Good choice of size by the way.  10" glass at f/6 behaves really well and you get a skookum scope.



#42 Chris Adamczyk

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Posted 04 December 2019 - 09:00 AM

I can work TOT, if I get a break in the rain tonight I'll try TOT. I tried the Foucault test last night and didn't see any difference between the Clouder zones and was guessing that was because I'm not that much different from a sphere to see any difference. My mark 2 tester is an improvement, I'll also work on using a camera on a stand, instead of a hand held phone.

 

Thank you

Chris



#43 dogbiscuit

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Posted 04 December 2019 - 02:05 PM

Ok...

 

For working TOT the top of the work surface needs to be flat.  Not optically flat to a few nanometers, but let's say more like carpenter flat, say to about 1/64" or better. The back of the mirror blank should be flat, and your blank being plate glass is almost certainly flat enough.  If the the mirror blank is placed on the work table there the two surface would contact on whatever remaining high areas on the two surfaces.  The mirror would flex over and between those points and cause the finished mirror to be misshaped.  A pad can be used as a buffer between the two surfaces to absorb the small remaining mismatches and allow reasonably flexure free work, or perhaps better said, will result in uniformly supported mirror blank that flexes in much the same way in all directions as you work different directions as you step around the mirror.  And just to be sure, after every trip or every few trips around the mirror, the mirror is rotated in increments so that any possible remaining asymmetry of the support, over time will have it's effect spread around the mirror.

 

 

The table also flexes.  Use a material that has equal strength in all directions.  Board lumber flexes differently along the grain than across the grain and so might not be as suitable as plywood or melamine.  And of course use sufficient thickness for the size mirror and work forces to resist flexure.   If the material might be deformed by moisture, protect it from water.


 

You are trying to make a surface accurate to a millionth of an inch or better.  As rigid as the mirror material seems, it does flex on the table, and as the lap is stroked over it.  It's important that the flexure is the same on all diameters as you work around the mirror. The mirror flexes more than that as you work.

 

TOT is not advisable without taking those precautions.

If you can do all that, here are some strokes that will work some correction into the outer zones.

The drawings show the path of the center of the lap over the mirror.

Only 1 turn of each TOT.

TwoThirdsW.jpg ThreeFourthsW.jpg


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#44 Chris Adamczyk

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Posted 04 December 2019 - 03:13 PM

My Work stand is 3/4" Plywood Painted to not soak up water, I'm using a 3/8" rubber mat always. If I read you correctly that is a of 2 total turns, 1 of each stroke style TOT.

 

Chris



#45 dogbiscuit

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Posted 04 December 2019 - 03:34 PM

My Work stand is 3/4" Plywood Painted to not soak up water, I'm using a 3/8" rubber mat always. If I read you correctly that is a of 2 total turns, 1 of each stroke style TOT.

 

Chris

Correct.

 

I'm editing this to add some of TOT concerns.

 

As best as reasonably possible the mirror should be centered on the work table so that as you walk around you have the same reach distance to the work.

 

The table should be level.

 

Pinching the mirror is bad. The mirror should not be tight between the cleats that hold it in position.  An extra 1/2mm to 1mm will assure the mirror is not pinched between the cleats.  It is ok if the mirror moves back and forth a little during the stroke.

 

Personal opinion... 4 cleats spaced 90° around the mirror is better than 3 cleats spaced 120°.

 

The pad should lay flat on the table and not be bunched up at the cleats.


Edited by dogbiscuit, 05 December 2019 - 12:17 AM.


#46 Chris Adamczyk

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Posted 05 December 2019 - 08:48 AM

Outside ROC TOT.jpg

Inside ROC TOT.jpg

 

I did the 2 rounds TOT, it looks like correction was added to the outer area. I supported the tool a little as it weighs 8-10LBs on the bathroom scale. From what I can tell the edge looks OK?



#47 dogbiscuit

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Posted 05 December 2019 - 11:50 AM

Don't support the lap for this.  10 pounds is not too heavy.

There may be some figuring needs where lifting some of the weight off the overhang is done, but not this.

The overhang is what adds the correction.

 

Palms and fingers down on the back of the lap, thumbs touching in the center, applying even pressure over all hand contact with the back of the lap.  As you stroke don't shift pressure to fingers or heel of palm alter that pressure during the stroke.  With the large strokes you might fear the lap will tip off the edge of the mirror.   With even pressure it will not fall over the edge if the center of the lap is inside the edge of the mirror. 

 

 

I think it looks better.  The edge looks pretty good. 

 

 

I'll take a closer look at that and suggest a stroke in a little while.  I have a couple of things to do first.



#48 dogbiscuit

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Posted 05 December 2019 - 12:19 PM

Same stroke but without supporting the lap.

Even pressure, let overhang of the lap do the work

 

1 turn TOT

TwoThirdsW.jpg

 

 

1 turn TOT

ThreeFourthsW.jpg


Edited by dogbiscuit, 05 December 2019 - 12:21 PM.


#49 Chris Adamczyk

Chris Adamczyk

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Posted 05 December 2019 - 09:56 PM

I did the two turns TOT, the tool was grabbing as I was stroking across the center of the mirror, it looks like the edge got turned, not sure which way, it looks like turned down?

 

Inside ROC TOT 19-1205.jpg

Outside ROC TOT 19-1205.jpg



#50 dogbiscuit

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Posted 06 December 2019 - 05:35 AM

I should have told you, any time the lap grabs for any reason other than slurry having dried too much quit working.  Don't work with a grabby lap.  If the slurry has dried to much add slurry or water.  How dry is too dry depends on humidity, hardness/softness and other qualities of the pitch, channels and pressing.

 

 

Two common causes of jerky motion or grabbing are channels closing, or insufficient pressing.  Verify that channels are not closing any where on the lap. Channel walls have a way of pinching closed, looking something like this )( at places along the channel.  It may be that most channels are ok, but it only takes one of these pinches to mess things up.  If you see these sort of pinches open them up to be the same width and depth as normal channels.

 

Then when channels have been made right, press.

If they were already right, press.

 

 

The goal in channeling and trimming is to make the channels the same width and depth over all of the lap.  About 1/4" wide and deep is a good size channel for a start.  Any parts of the channel narrower than 1/4" should be opened to 1/4".  Let parts of the channels wider than 1/4" close up to 1/4" with normal work and pressing.

 

I'm not sure the turn down is any worse than before.  It might just show better in the outside ROC Ronchigram because the focus is better than previous Ronchigrams.  The inside ROC Ronchigram doesn't show the turn down, but it is not focused well.  It will help if you have a camera with manual focus, and Ronchigrams focused well on the mirror.

 

I've found the most effective advice for turn down edge is for the lap to be fully the size of the mirror surface. Turn down usually equals mirror diameter minus lap diameter. of the lap is inside the mirror's edge.  Center the mirror on the lap and that area of the mirror's surface you see outside the lap's edge is what turns down.  If you trim the edge of the lap, don't make the lap diameter smaller than the mirror.   If the lap diameter is smaller than the mirror's diameter, don't trim the edge and let it grow to full diameter as you press and work.  The full size lap will make the edge better with normal working without need of special strokes.  Some special strokes might speed repair of tde

 

You did get some correction in the outer zone. Correction extends out almost to 80% zone, then lessens to the edge.

 

It might help in your interpretation of Ronchi to understand for any fixed Ronchi grating position

Insider ROC where band horizontal spacing is less is farther inside ROC, meaning longer ROC at that part of the mirror.

Outside ROC where band horizontal spacing is less is farther outside ROC, meaning shorter ROC.

 

So looking at your inside ROC Ronchigram the bands are farther apart at the mirror's center where the ROC is shortest and  come closer together toward the mirror's edge where the ROC is longest.

 

Tell me.

Is you lap the same diameter as the mirror?

What pitch you are using, and what is you working temperature.

 


 

To add correction mostly in the center.

1 turn MOT

ThreeFourthsW.jpg

 

 

 

To add correction in outer zone

1 turn TOT

ThreeFourthsW.jpg

 

 

Yes.. same 3/4 W,  MOT and TOT.

 

I don't know the conditions of your lap or working environment, so I don't know if your edge will get better or worse.

If the pitch is good and working temperature is suitable for the pitch, the channels are good and maintained in good condition, the lap is pressed well, the lap is the same size as the mirror, the edge will get better.  Maybe you don't know about the pitch or maybe you can't tightly control the working temperature.  The size of the lap, maintaining good channels, and pressing well before each figuring session is easy enough to control.




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